1. Field of the Invention
The present invention relates to a developing device incorporated in an image forming apparatus, such as a copying machine, a laser printer, a facsimile or the like, and more particularly relates to a developing device for developing an electrostatic latent image with a one-component type developer, the latent image being formed on an electrostatic latent image carrying body, such as a photosensitive body, a dielectric body, or the like.
2. Description of the Related Art
Generally, in an image formation apparatus, such as an electrophotographic recording apparatus, an electrostatic latent image is formed on an electrostatic latent image carrying body, such as a photosensitive body or a dielectric body; the latent image is electrostatically developed, as a charged toner image, with a developer; the charged toner image is electrostatically transferred from the electrostatic latent image carrying body to a recording medium, such as a sheet of paper; and then the transferred toner image is fixed on the recording medium by heat, pressure, light, or the like. As one type of developer, a two-component type developer is well known, which is composed of a toner component (fine particles of colored synthetic resin) and a magnetic component (fine magnetic carriers).
Usually, a developing device using this type of developer includes a vessel for holding the two-component type developer, wherein the developer is agitated by an agitator provided therein, to thereby cause the toner particles and the magnetic carriers to be subjected to triboelectrification, and thus the toner particles electrostatically adhere to each of the magnetic carriers. The developing device also includes a magnetic roller provided in the vessel as a developing roller, in such a manner that a portion of the magnetic roller is exposed therefrom and faces the surface of the electrostatic latent image carrying body. The magnetic carriers with the toner particles are magnetically adhered to the surface of the magnetic roller to form a magnetic brush therearound, and, by rotating the magnetic roller carrying the magnetic brush, the toner particles are brought to a nip zone, or developing zone, between the magnetic roller and the electrostatic latent image carrying body for development of an electrostatic latent image formed thereon. Namely, the magnetic component of the two-component type developer performs the two functions of electrically charging the toner component of the developer, and of bringing the toner particles to the developing zone.
The developing device using the two-component type developer has a good ability to bring the toner particles to the developing zone, which affects the quality of a developed toner image. Nevertheless, the ratio between the toner and magnetic components must be kept within a predetermined range before the ability to bring the toner particles to the developing zone can be ensured. Namely, since the toner component of the developer is gradually consumed during the developing process, a toner component should be periodically supplemented to the body of developer held in the vessel. Furthermore, when the magnetic component of the developer is deteriorated, the body of developer held in the vessel must be exchanged with a new one before a proper development of an electrostatic latent image can be obtained. In short, this type developing device has the disadvantages of a cumbersome control of a suitable component ratio in the developer, and a cumbersome exchange of a deteriorated developer with a new one.
Recently, a one-component type developer composed of only a toner component (fine particles of colored synthetic resin) has become favored because the disadvantages involved in the use of the two-component type developer can be eliminated by the use of the one-component type developer. Nevertheless, in the use of a one-component type developer, especially, a non-magnetic one-component type developer, it is difficult to decide how the toner should be electrically charged, and how the charged toner should be brought to a zone for developing an electrostatic latent image.
As a representative of developing devices using the one-component type developer, a known developing device comprises a vessel for holding the one-component type developer, a developing roller formed of a suitable conductive rubber material and provided within the vessel, and a developer-feeding roller formed of a suitable conductive sponge material and disposed in the vessel to be in engagement with the developing roller. A portion of the developing roller is exposed from the vessel, and faces an electrostatic latent image carrying body, such as a photosensitive drum, so as to form a developing zone therebetween. The developing roller and the developer-feeding roller are rotated in the same direction so that the surfaces thereof rub against each other in opposite directions at the nip therebetween. The rotating surface of the developer-feeding roller entrains toner particles and feeds the same to the nip between the developing roller and the developer-feeding roller, and the fed toner particles are subjected to triboelectrification so that they are electrically charged. The charged toner particles are electrostatically adhered to the rotating surface of the developing roller due to the image forces acting therebetween, to thereby form a developer layer therearound, and then the developer layer is brought to the developing zone for development of an electrostatic latent image formed on the photosensitive drum. The developer also comprises a doctor blade member resiliently pressed against the surface of the developer roller to even out the thickness of the developer layer to thereby ensure an even development of the latent image.
The above-mentioned conventional developing device using the one-component type developer involves various issues to be resolved, as discussed below:
(a) During the developing process, toner particles having a smaller diameter are preferentially consumed from the developer held in the vessel, resulting in variation in a quality of a developed toner image. In particular, the one-component type developer is composed of toner particles having various diameters. For example, when the one-component type developer is composed of toner particles having an average diameter of 8 .mu.m, this means that the developer includes smaller toner particles having a diameter of less than 8 .mu.m, and larger toner particles having a diameter of more than 8 .mu.m. In fact, a marketable one-component type developer having an average diameter of 8 .mu.m even includes toner particles having less than 5 .mu.m, and toner particles having a diameter of more than 10 .mu.m. When the toner particles are fed to the nip between the developing roller and the developer-feeding roller, the smaller toner particles are susceptible to triboelectrification, whereas the larger toner particles are not susceptible to triboelectrification. Accordingly, the smaller toner particles are more strongly adhered to the developing roller than the larger toner particles. When the thickness of the developer layer is regulated by the doctor blade member, the smaller toner particles, or sufficiently-charged toner particles, can pass a gap between the developing roller and the doctor blade, but the larger toner particles, or insufficiently-charged toner particles, are easily removed from the developing roller by the doctor blade member due to the weakness of the image forces acting therebetween. The removed toner particles are returned to the body of developer held in the vessel, and thus an average diameter of toner particles included therein gradually becomes larger. Thus, in the case where a new fresh developer is loaded in the vessel, a definition of an earlier-developed toner image is superior to that of a later-developed toner image, because very fine toner particles are used in the earlier-developed toner image. Namely, in the conventional developing device a constant definition or quality of a developed toner image cannot be ensured due to the preferential consumption of the smaller toner particles. PA1 (b) During the developing process, some toner particles are fused and stuck to the doctor blade member, and thus a proper regulation of a thickness of the developer layer cannot be ensured due to the toner adhered to the doctor blade member. In particular, although it is explained in Item (a) that the larger toner particles are removed from the developing roller, a very small part of the larger toner particles can pass the gap between the developing roller and the doctor blade member, but a majority of the toner particles passing the gap stay in a small space defined by the doctor blade member and the rotating surface of the developing roller, without being brought to the developing zone. The toner particles staying in the small space are fused due to the friction between the doctor blade member and the developing roller, and the fused toner is stuck to the doctor blade. PA1 (c) When an electrostatic latent image pattern, except for a character latent image, such as an alphabet image, a numerical image, a symbol image, or the like, is developed as a solid-image pattern or a halftone-dot-image pattern, a developing hysteresis appears in the developer layer formed around the developing roller. The developing hysteresis may be defined as an unevenness of a density of toner particles produced in the developer layer in accordance with a shape of a developed solid-image pattern or a developed halftone-dot-image pattern. The appearance of the developing hysteresis results in an occurrence of the earlier-developed image pattern as a ghost pattern in a later-developed toner image pattern. Of course, the occurrence of the ghost pattern deteriorates the quality of the later-developed toner image pattern. PA1 (d) When a local area of the developer layer does not participate in development of electrostatic latent images during the developing process, i.e., when any toner particles are not consumed from the local area of the developer latent image, the toner particles included in the local area are stuck as a film to the surface of the developing roller, because the toner particles concerned are repeatedly subjected to triboelectrification, and are strongly adhered to the developing roller with the large image force. A coefficient of friction of the toner-film-stuck surface of the developing roller becomes smaller, the toner particles cannot be sufficiently subjected to triboelectrification between the developer-feeding roller and the toner-film-stuck surface of the developing roller, and thus a sufficient amount of toner particles cannot be brought to the developing zone, to thereby deteriorate the quality of a developed toner image.